1. Technical Field
Embodiments of the present invention relate to an adhesive film for polarizing plates, a polarizing plate including the same, and an optical display including the same.
2. Description of the Related Art
A liquid crystal display includes a panel for liquid crystal displays, which includes liquid crystals, and a polarizing plate bonded to the panel via an adhesive layer. The polarizing plate has a multilayer structure including a polarizer and a protective film formed on the polarizer.
Since the polarizer and the protective film are formed of materials having different molecular structures and compositions, the polarizer and the protective film have different physical properties. In addition, the polarizer is stretched to have a unidirectional molecular arrangement so as to exhibit polarization. Thus, the dimensions of the polarizing plate may change due to shrinkage or expansion of the polarizer at high temperature and/or high humidity. In particular, when the polarizing plate is secured to the panel via an adhesive, stress resulting from shrinkage or expansion of the polarizing plate is concentrated on the protective film at high temperature and/or high humidity, so that birefringence occurs on the protective film, thereby causing light leakage.
Light leakage may be suppressed by preventing residual stress from remaining in the protective film through adjustment of stress on the adhesive layer. However, since the protective film has residual stress at high temperature and/or high humidity, a crosslinked polymer in an adhesive film is aligned in a specific direction, causing birefringence. Generally, an acrylic adhesive exhibits negative birefringence.
Relieving stress by reducing the storage modulus of the adhesive film for polarizing plates may suppress light leakage. However, while this method is effective for 7 inch or smaller displays, the adhesive is very flexible and thus may not be durable, (e.g., exhibiting bubbling or detachment) long-term or resistant to stress caused by high temperature and high humidity, and may therefore suffer from deterioration in processability, such as leakage of the adhesive. Alternatively, changes in retardation of the protective film at high temperature and high humidity can be reduced through suppression of stress by increasing the storage modulus of the adhesive film for polarizing plates. However, while light leakage may be suppressed in a central portion of the display screen, severe light leakage may occur at an end portion or tip end of the screen due to increasing stress with increasing distance from the center to the end of the screen.
An acrylic adhesive composition which includes an acrylic copolymer and a liquid optically anisotropic compound has been proposed containing a mesogen core in a structure thereof, and thus may adjust the negative birefringence of an adhesive layer due to residual stress to a positive birefringence while securing stress relaxation.
When a stress-relieving adhesive having positive birefringence is used, light leakage may be effectively suppressed at an end portion of the screen (at which portion changes in retardation of a protective film are large) under high temperature/high humidity conditions. In this case, light leakage in the central portion of the screen (at which portions of the protective film suffers almost no change in retardation) is increased.